U.S. Pat. No. 3,755,477 describes a process for producing fluorinated aliphatic hydrocarbons which comprises fluorinating a halogenated aliphatic hydrocarbon, including tetrachloroethylene and chlorotrifluoroethylene, by reaction in the gas phase with HF in the presence of a steam-treated and calcined chromium oxide catalyst prepared by a multi-step process. Example 23, column 5, shows tetrachloroethylene as a raw material with formation of CF.sub.3 CHCl.sub.2 (20%), CF.sub.3 CHClF (20%), CF.sub.3 CHF.sub.2 (30%), and CF.sub.3 CClF.sub.2 (20%) at 10/1 HF/C.sub.2 Cl.sub.4 mol ratio, 5.4 seconds contact time and 360.degree. C. reaction temperature. Example 24, column 5, shows chlorotrifluoroethylene as a raw material with formation of CF.sub.2 .dbd.CF.sub.2 (20%) and CF.sub.3 CHClF (13%) at 1.8/l HF/C.sub.2 ClF.sub.3 ratio, 4 seconds contact time and 320.degree. C. reaction temperature. In these examples, less desirable pentafluorinated products are obtained in a greater amount than the desired tri-and tetrafluoro products.
U.S. Pat. No. 3,258,500 describes a process for the catalytic vapor phase reaction of HF with halohydrocarbons, including tetrachloroethylene and chlorotrifluoroethylene, employing a catalyst that consists essentially of a heat-activated anhydrous chromium (III) oxide which may be supported on alumina. This catalyst is highly active. Example 17, column 14 showing fluorination of tetrachloroethylene with this catalyst, like that of the above '477 patent, produces large quantities of the less desirable highly fluorinated pentafluoroethane. At 400.degree. C. the product distribution is 35.0% pentafluoroethane, 9.2% 1-chloro-1,2,2,2-tetrafluoroethane, and 3.5% 1,1-dichloro-2,2,2-trifluoroethane. At 300.degree. C. the product distribution is 38.3% 1-chloro-1,2,2,2-tetrafluoroethane, 25.4% pentafluoroethane, and 16.0% 1,1-dichloro-2,2,2-trifluoroethane. Example 20, column 15, shows that chlorotrifluoroethylene yields CF.sub.3 CHF.sub.2 as the major product at 400.degree. C.
U.S. Pat. No. 3,591,646 discloses the use of catalysts, such as oxides of chromium, for preparation of chlorofluoroethanes by the catalytic reaction, in the gaseous phase, of acetylene, HF and chlorine in the presence of a recycle mixture of halogenated hydrocarbons. In column 2, lines 24-26, it is stated that the catalyst can be associated with modifiers which enhance its activity. Some examples of modifiers are fluorides of potassium, sodium, lithium and cesium. Claim 6 discloses a calcinated and fluorinated alumina catalyst which is impregnated with ammonium dichromate.
These catalyst are not entirely satisfactory in terms of fluorinating activity and catalyst life.